Exemplary embodiments relate to a memory device, and more particularly, relate to a data storage system including a multi-level memory device.
Semiconductor memory devices may be generally classified into volatile memory devices and non-volatile memory devices. Volatile memory devices may lose stored contents at power-off, while non-volatile memory devices may retain stored contents even at power-off. Nonvolatile memory devices may be formed of various types of memory cell transistors. Nonvolatile memory devices may include a flash memory device, a Ferroelectric Random Access Memory (FRAM) device, a Magnetic RAM (MRAM), a Phase change RAM (PRAM), or the like.
As a nonvolatile memory device, the flash memory device may be divided into NOR type flash memory devices and NAND flash memory devices according to connection relationships of memory cells and bit lines. The NOR flash memory device may have a structure in which two or more memory cell transistors are connected to one bit line in parallel. Accordingly, the NOR flash memory device may have excellent characteristics relating to random access time. On the other hand, the NAND flash memory device may have a structure in which two or more memory cells are connected to one bit line in series. This structure is called the cell string structure. One cell string may necessitate one bit line contact. Accordingly, the NAND flash memory devices may have excellent characteristics relating to integration.
Memory cells of flash memory device may be determined as ON cells or OFF cells according to a threshold voltage distribution. ON cells may represent erased cells, and OFF cells may represent programmed cells. A programmed memory cell may have one threshold voltage belonging to one of threshold voltage distributions each corresponding to N program states (or, programmed data values) (N being an integer of 1 or more).
At programming, the coupling effect may be caused between selected memory cells and adjacent memory cells. The coupling effect may make a threshold voltage distribution corresponding to the selected memory cells become wide and a margin between adjacent threshold voltage distributions become narrow. Such a coupling effect may be called the “electric field coupling” or “F-poly coupling”. If variation of a threshold voltage distribution corresponding to the selected memory cells and reduction of a margin between adjacent threshold voltage distributions are caused due to the coupling effect, it may be difficult or impossible to reliably read data from memory cells. This problem may increase in proportion to an increase in the number of data bits being stored per cell.